The best mattress for 5 years

We absolutely loved this king - sized mattress for 5 years but it started sagging - it would take extra effort to roll out of the indentations in the morning as we both woke up with aching backs. We could also feel the difference when one of us would sleep in the middle that was not worn down. We paid the required $35 for inspection & were told it wasn't bad enough to qualify for any compensation in the supposed 25 year warranty. We will never buy another Serta product.